This invention relates to paper coating pigment systems useful in producing printing grade papers. More particularly, this invention relates to lightweight polymer coatings, capable of producing printing papers of good ink receptivity and high gloss, obtainable under the relatively mild calendering conditions prevalent in standard paper machine finishing operations.
The principle purpose of a printing coating is to fill-in the voids between paper fibers and bury them below a bright surface capable of uniformly absorbing printing inks. Typical printing paper coatings comprise a pigment that forms the printing surface, a binder for securing the pigment to the paper web substrate, and a carrier or solvent that permits application of the coating to the substrate in a liquid form. Conventional high-gloss printing paper is manufactured by applying to the sheet aqueous emulsion coatings comprised of natural or thermoplastic binders and inorganic pigments. Common pigments are clays, titanium dioxide, and other white, bright, finely divided inorganic solids. Common binders are starches, casein, or latexes of copolymers and butadiene and acrylonitrile, vinyl acetate and the like. The carrier is generally water or other solvent that does not react with the pigment and is removable during the paper finishing processes by the application of heat.
To achieve the high gloss desirable for printing grade papers, a cast coating technique may be used in which the fluid coating, after being placed on the substrate, is contacted with a nonadhering surface having a high finish substantially equal to that of the required finished paper, whereupon the coating is dried into the nonplastic state. The problem with cast coating is that usually only slow machine speeds are possible and the tendency of coating weights to be in excess of 10 lbs./3,000 ft..sup.2, adding considerable cost to the finished paper.
A most desirable method of coating to achieve a printing grade paper would be to apply the coating while the web substrate is still on the paper machine. While it is possible to apply the coating to the web while it is on the paper machine, through transfer rolls and the like, it is generally not possible to obtain the desired gloss under the relatively mild calendering conditions that exist at the end of the paper machine. With standard inorganic coating pigments, the web generally must be supercalendered off of the paper machine to achieve adequate gloss and smoothness. Supercalendering is an additional operation which adds cost to the finished paper. Also, inorganic pigments are often brittle, subject to dusting or otherwise do not perform well under high calendering pressures.
As a means of solving some of these difficulties, a portion of the binder may be replaced with a thermoplastic material. For example, in Rice, U.S. Pat. No. 3,281,267, a coating containing kaolin clay or titanium dioxide is provided with a thermoplastic and elastomeric binder in addition to the usual casein or starch, to impart better coating flexibility at conditions necessary to achieve high gloss and opacity. Increasing the amount of thermoplastic in the pigment-binder mix, to effect a replacement of a portion or even all of the inorganic pigment is recognized by Heiser in U.S. Pat. No. 3,779,800 as improving gloss while other printing paper characteristics remain in the desirable range. Heiser notes, however, that careful control of temperature in drying or finishing by calendering or supercalendering is critical so that the temperature of the coating does not exceed the softening point of the thermoplastic. If the temperature or calendering pressure rises too high, the thermoplastic particles will tend to lose their discrete character and flow together, resulting in a rapid decrease in opacity and brightness of the coated paper as the plastic becomes more like a continuous transparent film.
A completely thermoplastic pigmented coating allows achievement of gloss under the relatively mild calendering conditions which exist at the end of the paper machine, but there remains the tendency of the coating to become transparent, as noted above, and results in lower opacities than if inorganic pigments are used. Thermoplastic pigments also tend to abraid from the paper substrate. Ink receptivity for thermoplastic coatings is generally poor as the particles are not readily wetted or absorbed by thermoplastic materials even when careful control of temperatures and calendering pressures is maintained to ensure their particulate characteristics.